1. Field of the Invention
The present invention relates to the field of live print scanning.
2. Background Art
Traditionally, techniques for obtaining a print image have included application of ink to a person's fingertips, for instance, and rolling or simply pressing the tips of the individual's fingers to appropriate places on a recording card. This technique can be very messy due to the application of ink, and may often result in a set of prints that are difficult to read.
Today, print scanning technology includes electro-optical devices for capturing images of a print pattern found on a biometric object, such as, a finger, a palm, a foot, etc. In such instances, the electro-optical device may be a fingerprint scanner, a palm scanner, or another type of biometric scanner. These scanners are also referred to as live print scanners. Live print scanners do not require the application of ink to a person's finger or palm. Instead, live print scanners may include a prism located in an optical path. A platen is used as the surface for receiving the biometric object. For example, with an optical fingerprint scanner, a finger is placed on the platen, and a camera detects an image of the fingerprint. The platen can be a surface of the prism or any other surface provided on the prism and in optical contact with the prism. The fingerprint image detected at the camera is comprised of relatively light and dark areas. These areas correspond to the valleys and ridges of the fingerprint.
Live print scanners typically utilize the optical principle of total internal reflection (TIR). The rays from a light source internal to these optical scanners reach the platen at an incidence angle that causes all of the light rays to be reflected back. This occurs when the angle of incidence is equal to or greater than the critical angle, which is defined at least in part by the ratio of the two indices of refraction of the medium inside and above the surface of the platen.
In the case of a live fingerprint scanner, one or more fingers are placed on the platen for obtaining a fingerprint image. In a TIR live print scanner, ridges on a finger operate to alter the refraction index at the platen, thereby interrupting the TIR of the prism. This interruption in the TIR causes an optical image of the ridges and valleys of a fingerprint to be propagated through the receiving surface and captured by a camera internal to the device.
In many live scan applications, illumination intensity needs to be even across a platen surface to allow capture of a high-quality image. Individual light emitters, however, can have an uneven light distribution. For example, an array of light emitters has an uneven light distribution due to its discrete light sources. A fluorescent tube also has an uneven light distribution across its length. There is a need for systems capable of illuminating an area of the platen with a diffuse uniform light.